Fluid powered motors driving an additive injection pump connected to a source of fluid additives are typically installed in a line containing primary fluid under pressure. The primary fluid produces reciprocating movement of a piston assembly within a housing of the fluid motor. The fluid motor in turn reciprocates a piston within a cylinder of the additive injection pump to draw a quantity of secondary fluid into the primary fluid. Such devices have been applied to add medication to drinking water for poultry and livestock, treat water with additives, add fertilizer concentrate to irrigation water, or add lubricant or cleaning agents to water. In liquid additive injection pumps, such as that shown in U.S. Pat. No. 4,558,715, reciprocating movement of the piston assembly is produced by a valve mechanism operable to establish a differential pressure. Specifically, opening and closing of the valve mechanism synchronized to the upstroke and down stroke positions of the piston assembly produces a pressure differential that moves the piston through its reciprocating cycle. Opening and closing of the valve mechanism is synchronized to the piston assembly by an over-center mechanism, which is actuated coincident with the piston assembly reaching the ends of its upstroke and down stroke positions. The over-center mechanism is spring-biased and serves to toggle the valve mechanism open and closed when an actuating shaft carried by the piston assembly engages stops that define the ends of its upstroke and down stroke excursions.